Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5 wt% glass content becomes significant from sintering temperature of 1000 °C. The glass content shows a strong influence on the Curie temperature T_c, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba₂TiSi₂O₈. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb²⁺ in Ba²⁺ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10 wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1 kV/mm was achieved for BST ceramics with 10 wt% glass content sintered at 900 °C.     

Microstructure and electrical properties of Mn/Y codoped Ba0.67Sr0.33TiO3 ceramics

Pure and Mn/Y codoped Ba_0.67Sr_0.33TiO₃ (BST) ceramics were fabricated via the citrate–nitrate combustion technique, and the microstructure and electrical properties of BST ceramics were mainly investigated. The Mn/Y codoping concentration has a strong influence on the microstructure and electrical properties of BST ceramics. All BST ceramics possess a pure polycrystalline structure. The density, dielectric loss, leakage current, and ferroelectric properties are improved by codoping 0.5 mol% Mn and 1.0 mol% Y to BST. The relative density of 0.5 mol% Mn/1.0 mol% Y-codoped BST (BST0510) ceramics reaches 97.5% of the theoretical value. BST0510 ceramics have the lowest dielectric loss (tanδ < 0.0073 at 1 kHz) among all BST ceramics. BST0510 ceramics also demonstrate a low leakage current density (1.23 × 10^?7 A/cm²) at an applied field of 10 kV/cm, and excellent ferroelectric properties with a remanent polarization of 2P_r = 15.327 μC/cm² and a coercive field of 2E_c = 3.456 kV/cm. Therefore, the Mn and Y with optimum content help improve the electrical properties of BST materials.     

Ferroelectric Phase Transition,Interfaces Quality,and Stress Evolution of TiOx/BaxSr1−xTiO3 Structures

Ba(Sr,Ti)O₃ material presents a remarkable property that lies in the possibility to change the permittivity by applying a dc electric field, i.e., BST is a tunable material. That makes BST a very interesting material for the development of reconfigurable devices in microelectronics. In this study, we focus our work on Ba(Sr,Ti)O₃ with Ba/Sr = 30/70, the films are deposited by radio‐frequency magnetron sputtering on Al₂O₃ (0001). A buffer layer of TiO_x is used to control the film orientation. The influence of this buffer layer on the dielectric properties, the interfaces quality with respect to the film thickness, and the temperature is analyzed. An increase of 30% of the relative permittivity was measured and a tunability of 50% was attained at 300 KV/cm. The dielectric measurements on BST/TiO_x as a function of the temperature show a shift of the Curie temperature (T_c = ?40°C) in comparison to BST without TiO_x layer (T_c = ?80°C). We demonstrate that the Curie temperature does not correspond to the maximum permittivity. The important stress measured on the films (930 MPa) could explain this behavior.     

Influence of solid loading on densification behavior,micromorphology and dielectric properties of Ba0.67Sr0.33TiO3 ceramics fabricated by a novel DCC-HVCI method

Ba_0.67Sr_0.33TiO₃ (BST) ceramics with highly improved dielectric performance were fabricated by a novel direct coagulation casting via high valence counter ions (DCC-HVCI) method. The influence of solid loading on densification behavior, micromorphology, and dielectric performance of the samples was investigated. With the increase of solid loading from 40 to 50 vol%, the maximum densification rate of BST ceramics increased from 0.090 to 0.122 s⁻¹, and the densification temperature decreased from 1424 to 1343 °C, which indicated that high solid loading could promote the densification behavior of samples during sintering. BST ceramics fabricated by the DCC-HVCI method showed uniform grain size and microstructure, which was beneficial for the dielectric properties of BST ceramics. Samples obtained from 45 vol% suspensions possessed the lowest dielectric permittivity (ε_r ≈ 2801), and the dielectric loss (tanδ≈0.0262) was about 1/10 of that of dry-pressed samples (tanδ≈0.301), which could be attributed to the composition homogenization.     

Effect of grain size on the energy storage properties of (Ba0.4Sr0.6)TiO3 paraelectric ceramics

(Ba_0.4Sr_0.6)TiO₃ (BST) ceramics with various grain sizes (0.5–5.6 μm) were prepared by conventional solid state reaction methods. The effect of grain size on the energy storage properties of BST ceramics (T_c ≈ −65 °C) was investigated. With decreasing grain sizes, a clear tendency toward the diffuse phase transition was observed and the dielectric nonlinearity was reduced gradually, which can be explained by the Devonshire's phenomenological theory (from the viewpoint of intrinsic polarization). Based on the multi-polarization mechanism model, the relationship between the polarization behavior of polar nano-regions (the extrinsic nonlinear polarization mechanisms) and grain size was studied. The variation of the grain boundary density was thought to play an important role on the improvement of dielectric breakdown strength, account for the enhanced energy density, which was confirmed by the complex impedance spectroscopy analysis based on a double-layered dielectric model.     

High-temperature 0.5BiFeO3–0.5PbFe0.5Nb0.5O3 multiferroic: Microstructure,ferroelectric properties,and Mössbauer effect

Research findings of the microstructure, dielectric, ferroelectric characteristics, and Mössbauer effect of solid solution ceramics with 0.5BiFeO₃–0.5PbFe_0.5Nb_0.5O₃ composition in a wide temperature range are presented. The examined ceramic chip surface allows one to draw conclusions about the internal homogeneity of grains and the absence of pores inside them. It was shown that Fe³⁺ iron cations in the material are valence and they are found only in seven locally different states, which is associated with disorder in the solid solution structure. The Néel temperature is T_N ~ 445 K. The anomalous behavior at T < 30 K becomes clear when analyzing the dielectric spectra of 0.5BiFeO₃–0.5PbFe_0.5Nb_0.5O₃ ceramics in the range of 10 … 1000 K. It is explained by the appearance of a spin-glass state in the object. The presence of contributions to the dielectric response in ceramics at T > 300 K is revealed. It is claimed that the ferroelectric–relaxor → paraelectric phase transition caused the low-temperature contribution, and the second one is a manifestation of the Maxwell–Wagner polarization and the corresponding non-Debye type dielectric relaxation. The causes of the revealed regularities and the prospects for using the material in the thin films form are discussed.     

Effects of Nb-doping on the micro-structure and dielectric properties of (Bi0.5Na0.5)TiO3 ceramics

Bismuth sodium titanate [(Bi_0.5Na_0.5)TiO₃ or BNT] ceramics incorporated with 0, 1, 5, 10, 15 and 20 mol% niobium were prepared by conventional solid state reaction method. The green bodies were sintered at 1050 °C for 2 h to obtain dense ceramics. The effects of substitution of niobium ion for titanium ion in BNT ceramics on micro-structure and dielectric properties were investigated. X-ray diffraction analysis showed the presence of a secondary phase when more than 5 mol% niobium was added. Within the solubility limit, Nb doping caused the grain size of BNTNb to be smaller than the undoped sample. The investigation of the dielectric properties showed that the transition temperature (T_c) was found to shift towards lower temperature as the content of Nb increased. In this research, the donor-type behavior and induced charged defects had significant influence on the electrical properties of Nb-doped BNT ceramics.     

Structural and dielectric properties of B2O3/Li2O-added Ba0.6Sr0.4TiO3 multilayer ceramics for tunable devices

B₂O₃ and Li₂O (B/L)-added Ba_0.6Sr_0.4TiO₃ (BST) ceramics sintered at 940 °C exhibited a dense microstructure with large grains. The amount of B/L additive was 4.5 wt% with a B/L ratio of 1.5:1. The B/L-related liquid phase assisted the densification of the BST ceramics. This BST ceramic displayed a large dielectric constant (ε_r) of 2834 with a low dielectric loss (tan δ) of 0.21% at 1.0 MHz. It also displayed a large tunability (28.2% at 10 kV/cm) and a high figure of merit (FOM) of 134. BST thick-films were synthesized using the tape casting method. The thick-film densified at 940 °C exhibited a large tunability of 18.7% at 10.0 kV/cm and an FOM of 208; these are higher than the values reported in the literature. Multilayer ceramics (MLCs) consisting of five layers of 40-μm-thick BST thick-films and Ag electrodes were also fabricated at 940 °C. No diffusion occurred between the Ag electrode and BST thick-film. A large tunability of 67.6% at 52 kV/cm with a high FOM of 294 was obtained from this MLC. This verified that the B/L-added BST ceramic is effective for application in tunable multilayer devices.     

Noncontact temperature-dependent fluorescence depicting phase transition in Nd3+-doped (K0.5Na0.5)NbO3 ceramics

A noncontact temperature measurement technique based on fluorescence variation was used to depict the temperature-dependent evolution of phase transition of a ferroelectric material, that is, Nd³⁺-doped (K_0.5Na_0.5)NbO₃ ceramics. The slope of the fluorescence intensity curve changes dramatically in the two temperature regions of 450-475 K and 650-675 K, which correspond to orthorhombic-tetragonal and tetragonal-cubic transitions as confirmed by the temperature dependence of dielectric constant. Furthermore, the small deviations in δT_O-T and δT_c indicate the good accuracy of this noncontact method. This work can guide other ergodic ferroelectrics to describe phase experience by the noncontact fluorescence method.     

Improved energy storage performance of Bi(Mg0.5Ti0.5)O3 modified Ba0.55Sr0.45TiO3 lead-free ceramics for pulsed power capacitors

(1–x)Ba_0.55Sr_0.45TiO₃–xBi(Mg_0.5Ti_0.5)O₃ (x = 0, 0.08, 0.1, 0.12, 0.15, 0.2) ceramics were fabricated via a solid-state reaction route. The ultrahigh recoverable energy density (W_rec = 4.05 J cm^?3), efficiency (η = 78%), maximum polarization (P_max = 51.40 μC cm^?2), and high dielectric breakdown strength (BDS = 230 kV cm^?1) were achieved for the 0.9BST?0.1BMT ceramic. The fast discharge rate (t_0.9～0.14 μs), current density (C_D～637.02 A cm^?2), high power density (P_D～38.70 MW cm^?3), good temperature stability (20?180 °C), frequency stability (10?500 Hz), and fatigue endurance for cycling (10⁵) of 0.9BST?0.1BMT ceramic make it suitable for the development of energy-storage devices. The relaxor behavior with a high W_rec (3.06 J cm^?3) and η (93%) at BDS (220 kV cm^?1) was also achieved for the 0.8BST?0.2BMT ceramic. This study systematically investigates the correlation among the structural, dielectric, impedance, and energy storage properties of BMT-doped BST ceramics.     

Raman and dielectric spectroscopic analysis of magnetic phase transition in Y(Fe0.5Cr0.5)O3 multiferroic ceramics

Here, we report the Raman and dielectric spectroscopic studies as a function of temperature of orthorhombically distorted Y(Fe_0.5Cr_0.5)O₃ (YFC) ceramics, measured from 80 to 300 K. The dc-magnetization measurements under field cooled (FC)-zero field cooled (ZFC) protocol indicate a small onset of magnetic ordering at T_N∼270 K. The field dependent magnetization plot recorded at 50 K, 150 K and 200 K show a clear opening in hysteresis loops. The linear dependence of magnetization plot at high field without any saturation of magnetization indicates the coexistence of weak ferromagnetic (WFM) component within the canting antiferromagnetic (CAFM) matrix. Temperature evolution of Raman line-shape parameter of B_2g(4) phonon mode clearly exhibits an anomalous behavior of phonon shift near T_N∼270 K, indicating the spin-phonon coupling in the ceramics. From the temperature dependent dielectric permittivity (ε(T)) study, two dielectric relaxation peaks are detected below 200 K and above 250 K. The appearance of former relaxation peak is responsible for polaronic conduction mechanism, while the later one is associated with magnetic phase transition which might be relevant to the presence of magnetoelectric coupling in YFC ceramics. The observed P-E hysteresis loops at room temperature indicate weak ferroelectric nature of the ceramics.     

Enhanced Dielectric Properties of Ba0.5Sr0.5TiO3/MgO Composites Prepared by Coprecipitation Derived Core–Shell Powders

Ba_0.5Sr_0.5TiO₃/MgO (BST/MgO) composite ceramics have been prepared in situ by using an oxalate precipitation process. SEM and TEM images show that oleic acid modified BST/MgO nanocomposite exhibits helminth‐like particles consisting of BST core and MgO shell. Results reveal that oleic acid modification is chemisorbed on MgO particles as a carboxylate to form MgO shell, which connects to interparticles. Accordingly, the BST particles are well surrounded naturally by a thin MgO layer in the composite ceramics. As a result, the oleic acid modified BST/MgO composite ceramics shows better dielectric properties, with permittivity of 203, loss tangent of 0.0025 (at 2.4 GHz), and tunability of 10.2%.     

Effect of layered structure on dielectric properties and energy storage density in xBa0.7Sr0.3TiO3-SrTiO3 multilayer ceramics

xBa_0.7Sr_0.3TiO₃-SrTiO₃ (BST-ST) multilayer ceramics with different BST layers (x=1, 3, 5) were designed and fabricated by lamination of Ba_0.7Sr_0.3TiO₃ (BST) and SrTiO₃ (ST) tapes. Dielectric and energy storage properties of the multilayer ceramics were investigated. BST-ST multilayer ceramics exhibited enhanced temperature- and frequency-stability of dielectric properties, accompanying high permittivity (~2000) and low dielectric loss (<0.005) at room temperature. P-E loops revealed that BST-ST multilayer ceramics displayed low remnant polarization and favorable maximum polarization. The optimal energy storage performance was obtained in the composition of x=5 with dielectric breakdown strength of 220 kV/cm and energy storage density of 2.3 J/cm³. These results indicate that BST-ST multilayer ceramics can be a favorable candidate for dielectric capacitor applications.     

Ferroelectric–Dielectric Composites Model of Ba0.5Sr0.5TiO3/Mg2AO4(A = Ti,Si) for Tunable Application

The correspondence between the theoretical model and the experimental results of the dielectric response in two‐phase composites of Ba_0.5Sr_0.5TiO₃ and Mg₂AO₄ (A = Ti, Si)was studied. The Ba_0.5Sr_0.5TiO₃ (BST50)/Mg₂AO₄ composites in 2‐2 model structure consisting of BST50 layers and Mg₂AO₄ layers were fabricated by tape casting and multilayer technique. The 3‐0 model of the two‐phase composites is fabricated by conventional ball mill mixing and solid‐state reaction process. The ceramics samples with dense structure were obtained because the coefficient of thermal expansion (CTE) of Mg₂SiO₄ (12.84 ppm/°C) and Mg₂TiO₄ (12.11 ppm/°C) ceramic specimens are close to the pure BST50(13.15 ppm/°C) ceramic. The microstructure, dielectric, and tunable properties of 2‐2 and 3‐0 model composites were investigated. The experimental results agree well with the theoretical prediction in 2‐2 model. An important feature of 2‐2 model composites is that the DC field is efficiently applied to the high‐permittivity ferroelectric phase. With the increase in Mg₂AO₄ volume fraction q, the tunability of the composite remains almost unchanged whereas the permittivity greatly reduced in the 2‐2‐//model. These results show that the 2‐2‐//model sample is good candidates for the tunable devices.     

Dielectric and ferroelectric properties of (1 − x)BaTiO3–xBi0.5Na0.5TiO3 ceramics

Lead-free (1 − x)BaTiO₃–xBi_0.5Na_0.5TiO₃ (x = 0.01, 0.02, 0.05, 0.1, 0.2, 0.3) ferroelectric ceramics were fabricated by the conventional ceramic technique. Sintering was made at 1200 °C for 2–4 h in air atmosphere. The crystal structure was investigated by X-ray diffraction. The dielectric and ferroelectric properties were also studied. Room temperature permittivity was found to decrease as Bi_0.5Na_0.5TiO₃ (BNT) content increases. Only the sample with 0.3 mol BNT was found to have relaxor behaviour. The T_c shifted slightly only for BNT addition lower than 0.1 mol. The highest T_c (about 150 °C) was obtained for 0.2 mol BNT addition. The remanent polarization, P_r, decreases whereas the coercive field, E_c, increases monotonously as the BNT content increases.     

Effect of the (Ba + Sr)/Ti ratio on the microwave‐tunable properties of Ba0.6Sr0.4TiO3 ceramics

The impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave‐tunable characteristics of diffuse phase transition (DPT) ferroelectric Ba_0.6Sr_0.4TiO₃ (0.6‐BST) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e., and . The magnitude of the dielectric constant, ε′, at room temperature in the absence of an applied electric field was governed by the shift in the dielectric maximum temperature, T_m, because T_m was close to room temperature for the 0.6‐BST. The dielectric loss, tanδ, diminished as the ε′ decreased for 0.98≤A/B≤1.05, while the tanδ was much higher for A/B=0.95 having the greatest A‐site vacancy loading. The negatively charged and were mainly compensated by oxygen vacancies and likely partly compensated by holes, h^?, which contributed to the electrical conduction. The tunability, T, at 100 MHz was almost constant at 20%–25% for A/B≥1.00 despite the reduction of the ε′, whereas T decreased for A/B<1.00 to ca. 10% for A/B=0.95 having the greatest A‐site vacancy loading. The results implied that the for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions (PNRs) than the for smaller A/B values, thereby providing greater dipole polarization. Consequently, the figure of merit, FOM, reached its maximum of 250 at A/B=0.9875, which was ca. 155% higher than that of the stoichiometric BST.     

Low-temperature synthesis of K0.5Na0.5NbO3 ceramics in a wide temperature window via cold-sintering assisted sintering method and enhanced electrical properties

High temperatures (≥ 1100 °C) and narrow temperature window (～ 20 °C) for sintering dense K_0.5Na_0.5NbO₃ ceramics always deteriorate their electrical properties. Here, via cold-sintering assisted sintering method, dense K_0.5Na_0.5NbO₃ ceramics were obtained in a wide temperature span between 800 °C and 1000 °C. An aqueous solution of NaOH and KOH mixture was used as transient liquid. Effects of liquid content (LC), molar concentration (MC) of liquid, cold-sintering temperature (T_CS), and post-annealing temperature (T_AN) on densification and electrical properties of the ceramics were investigated in detail. The ceramics prepared using LC = 10 wt%, MC = 10 mol/L, T_CS = 350 °C, and T_AN = 900 °C exhibit excellent electrical properties with d₃₃ = 123 pC/N, ε_r = 609, tanδ = 0.021, P_r = 28.0 μC/cm², P_m = 39.2 μC/cm², and E_c = 20.3 kV/cm. Compared to the ceramics with same or similar compositions via conventional solid-state sintering, the present K_0.5Na_0.5NbO₃ ceramics exhibit excellent electrical properties. The study endows the cold-sintering assisted sintering the successful method to prepare K_0.5Na_0.5NbO₃ ceramics at low temperatures and in a wide temperature window.     

Thermally-induced local structural transformations in Na0.5Bi0.5TiO3-KNbO3 ceramics

The local symmetry of (1-x)Na_0.5Bi_0.5TiO₃-xKNbO₃ (NBT-xKN, x = 0.01?0.07) ceramics was thoroughly examined using high-energy synchrotron x-ray scattering and analysed using the pair distribution function (PDF) method. At room temperature, the structure of NBT-xKN (x = 0.01?0.07) ceramics was best refined using monoclinic C1c1 structures within the local-range (r<22 Å) and rhombohedral R3c within medium-range (22 Å <r <52 Å). Furthermore, the local monoclinic C1c1 structure exists from room temperature to > 420 °C (above the temperature of the maximum dielectric peak, T_m), as confirmed by in-situ temperature-dependent PDF analysis.     

Nanocrystalline Barium Strontium Titanate Ceramics Synthesized via the “Organosol” Route and Spark Plasma Sintering

Dense nanocrystalline barium strontium titanate Ba_0.6Sr_0.4TiO₃ (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950°C and 1050°C starting from nonagglomerated nanopowders (~20 nm). The powders were synthesized by a modified “Organosol” process. X‐ray diffraction (XRD) and dielectric measurements in the temperature range 173–313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature T_C decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity ε shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%–4% in the frequency range of 100 Hz–1 MHz and temperature interval 160–320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer‐sized ceramics.     

Enhanced piezoelectric properties of Aurivillius-type sodium lanthanum bismuth titanate (Na0.5La0.5Bi4Ti4O15) by B-site manganese modification

The Aurivillius-type bismuth layer-structured ferroelectrics (BLSFs) sodium lanthanum bismuth titanate (Na_0.5La_0.5Bi₄Ti₄O₁₅, NLBT) polycrystalline ceramics with 0.0–0.4 wt% MnO₂ were synthesized using conventional solid-state processing. Phase analyses were performed by X-ray powder diffraction (XRPD), and the microstructural morphology was assessed by scanning electron microscopy (SEM). The dielectric and piezoelectric properties of the manganese-modified NLBT ceramics were investigated in detail. The results show that manganese is very effective in promoting the piezoelectric activities of NLBT ceramics, and the reasons for piezoelectric activities enhancement by manganese modification are explained. The NLBT ceramics modified with 0.2 wt% MnO₂ (NLBT-Mn2) possess good piezoelectric properties, with a piezoelectric coefficient d₃₃ of 28 pC/N. This value is the highest value among the modified NLBT-based piezoelectric ceramics examined. The temperature-dependent dielectric spectra show that the Curie temperature T_c of the manganese-modified NLBT ceramics is slightly higher than that of the pure NLBT ceramics. Thermal annealing analysis revealed that the manganese-modified NLBT ceramics possess good thermal stabilities up to 500 °C. These results demonstrate that the manganese-modified NLBT ceramics are promising materials for high temperature piezoelectric applications.